The proposed study is the first phase of a multi-center, randomized clinical trial, designed to assess the efficacy of reduction of total fat content in the diet in preventing breast cancer. Restriction of total fat content to 20% of total calories will be the main dietary intervention strategy among women ages 45 to 70 years who are at high risk for development of breast cancer. Qualifying criteria are as follows: 1) a first-degree relative with breast cancer and 2) any one of the following: (a) another first-degree relative with breast cancer, (b) one or more previous breast biopsies, (c) first pregnancy after age 30, (d) never pregnant. The first phase of this trial will test the feasibility of a full-scale trial which will involve 200 participants per center over a period of approximately five years for surveillance of the primary outcome event--breast cancer. Secondary analyses of interest include further definition of subgroups at special risk for development of breast cancer and subgroups likely to adhere and respond to dietary intervention; sequential changes in breast parenchymal patterns (mammography); and dietary modification of lipid subfractions. The trial will be carried out in three stages: 1) completion of the protocol and organizational development, 2) implementation of the protocol at the three institutions to document protocol adherence in the treatment and control groups, 3) implementation of the full-scale study, if warranted.